This invention relates to optical pump coupling of pump light, such as from a light source, to an optical guide, such as an optic fiber, and more particularly to highly efficient coupling of pump light from a multimode fiber into the inner cladding of a double clad fiber (DCF).
The use of double clad fibers (DCFs) have increasingly become important in optical applications requiring higher powers of amplified light or high power fiber lasers. The advent of DCFs has been known in the art for many years starting, for example, with U.S. Pat. No. 3,808,549 to Maurer, issued Apr. 30, 1974. In this patent, a double clad fiber is shown with an inner cladding being pumped with multiple LED sources, although it is clear from the disclosure that these pump sources may also be semiconductor laser sources, such as AlGaAs laser diodes pumping a neodymium doped core of the DCF. Thus, this patent represents one of the earliest disclosures of the concept relating to end pumping of a DCF with pump light from plural semiconductor laser sources.
As is well known in the art, DCFs have a single mode core with a refractive index n1 and having a diameter of several microns, such as approximately 6 xcexcm, or a multimode core with a refractive index n1 and having a diameter such as 10 xcexcm or 12 xcexcm, either of which may be doped with a rare earth active material. A single mode core provides for single mode propagation of a light signal beam in optical communication systems. A first or inner cladding with a refractive index n2, which may be of several tens or hundreds of microns, such as approximately 200 xcexcm, (sometimes referred to as a pump core and comprising, for example, silica glass, fluoride glass or ZBLAN) surrounds the fiber core and receives pump light from one or more pump light sources for multimode propagation of the pump light. A second or outer cladding with a refractive index n3, wherein n1 greater than n2 greater than n3, surrounds the inner cladding, which may be comprise of a polymer, and confines the pump light to the inner cladding.
Many advantages arise from the monolithic nature of a DCF laser system in that all components of the system are linked by fiber, rendering them immune to mechanical misalignments or contamination of the optical path, such as from noise and optical losses. The critical links in these systems are at the ends of the fiber where the pump source light or injection source light are coupled into the fiber, and at the transmitter head, where the light is launched into a transmitter telescope to converge the light, via an optical guide into the fiber, such as illustrated in U.S. Pat. No. Re. 33,722 to Scifres et al. At the pump end of such a system, one technology currently employed for coupling light into the fiber is to employ a fiber coupled laser diode bar with the individual outputs of the individual laser emitters of the bar are individually coupled to a fiber and the individual fibers are bundled into a larger aperture, such as through stacking as illustrated in U.S. Pat. No. 5,268,978 to Po et al. and in U.S. Pat. No. Re. 33,722 to Scifres et al., or are fused together as, into a multimode fiber as illustrated also in U.S. Pat. No. Re. 33,722 to Scifres et al. Moreover, their combined output may then be coupled into a multimode fiber, or a telescoped output or fused array of fibers that is a multimode output that is imaged onto the inner cladding of the DCF.
Other current technology for coupling pump light beam and a signal light beam into either a single mode fiber or a double clad fiber, functioning as a fiber amplifier or a fiber laser, comprises free-space optics a dichroic beamsplitter and lens system, as illustrated in FIG. 1. In this scheme, a signal to be amplified from single mode fiber 18 is coupled into a DCF amplifier 19 by a conventional lens system 15A and 15C. The signal beam 18A is transparent to and passing through dichroic mirror 17 into the core of DCF amplifier 19. Pump source 11 comprises a laser diode array or bar 12 where the output emission from the plural emitters are each respectively coupled into a fiber 13. Fibers 13 are bundled at their output ends and the bundled fibers are coupled into a single output, such as optical medium 14, which may be, for example, a multimode fiber. In order to achieve higher pumping powers, multimode laser sources with their combined multimode outputs into a single beam is preferred over other sources. The combined output of pump light 14A is collimated by lens 15B and is reflected by dichroic mirror 17 into the inner cladding of DCF amplifier 19.
While this standard coupling scheme can be made small and self-contained, it presents a potential mechanical weakness in the overall optical system and has relatively high optical losses for the signal beam propagating in the fiber because it is coupled out and then back into the transmission fiber. Taking the case of a DCF as amplifier 19, the coupling of pump light 14A into the inner cladding of the DCF is relatively tolerant since it constitutes a multimode coupling. However, the coupling of signal light beam 18A into the inner core of the DCF is more sensitive to misalignment, since it constitutes a single mode coupling. This is a critical factor. As a general principle, any diffraction limited laser system is a single spatial mode system, whether in fiber or in free space; and the alignment tolerances on single mode systems are severe. The advanced functionality of modern laser systems, particularly multi-stage systems, require that at each stage, the signal is mode matched to the next stage. Fiber laser systems are no exception. However, a fiber laser offers an opportunity to employ commercial fusion splicing since single mode fibers can be spliced together with negligible insertion loss, typically about 0.1 dB to about 0.25 dB. Once spliced, a single mode fiber connection is permanent, provides low loss and is immune to misalignment and contamination.
Breaking the continuity of an injection source single mode fiber 18 in order to inject the multimode pump light into amplifier 19 unnecessarily introduces a free-space single mode coupling into the optical system. In other words, in order to achieve end pumping of DCF amplifier 19, a spaced separation of single mode fiber 18 apart from the end of the DCF is necessary to launch the pump light into the inner cladding of DCF amplifier 19. Consequently, it would be highly desirable to develop a scheme to couple pump light such as from a multimode fiber into a double clad fiber without interrupting the light path of signal beam 18A into the single mode core.
This is not to say that others have not tried to achieve a similar function of coupling a single mode signal source into the core of a DCF and a multimode pump source into the cladding of a DCF without significant loss. U.S. Pat. No. 5,170,458 to Aoyagi et al. suggests such an optical coupler at 46 in FIG. 3 of the patent involving end pumping of a DCF. In FIG. 3 of that patent, coupler 46 is shown as a box with two inputs, one for a signal light 6 via single mode fiber 21 and another for pump light 38 from a semiconductor source 18 via multimode fiber 48. The single output from coupler 46 indicates direct coupling of signal light 6 to a core 52 of double clad fiber 50 and direct coupling of the pump light 38 into inner cladding 54 of fiber 50. In the disclosure, it is indicated that coupler 46 is xe2x80x9cof a well-known typexe2x80x9d, but those skilled in this art, particularly at the time of this disclosure in 1990, were not readily familiar with a single mode/multimode fiber coupled input to a double clad fiber, at least one that had high optical coupling efficiency and was readily available for successful commercial applications. Thus, there is no disclosure in this patent as to how such a coupler should be designed to provide for high efficient light coupling without introducing free-space single mode coupling into the optical system and without some interruption of the core of the signal fiber, whether single mode fiber or DCF.
It is also of interest to note that U.S. Pat. No. 5,170,458 to Aoyagi et al. in FIG. 2 discloses a scheme for side pumping of a DCF from multiple sides of the DCF. The scheme shown is free-space coupling of laser light from plural sources directly into the side of the inner cladding of the DCF. However, this type of coupling does not take full advantage of the numerical aperture (NA) of the fiber inner cladding so that the coupling efficiency is low. On the other hand, U.S. Pat. No. 4,815,079 to Snitzer et al. discloses DCF configurations and illustrates three designs in FIGS. 3-5. of side coupling of a multimode fiber 25 to a DCF 20. Since, here, there is direct coupling of the pump light into the inner cladding, particularly as illustrated in FIG. 3, this approach is likely more efficient than that shown in U.S. Pat. No. 5,170,458. However, these three designs only permit pump light to enter the inner cladding of the DCF from one side of the cladding. Also, in the case of the designs of FIGS. 4 and 5, the NA for the light has to be less than the NA of the fiber so that full advantage of the fiber NA is not being taken into consideration and, therefore, the coupling efficiency is lower. Therefore, less light is coupled into the DCF for pumping applications whereas the desire is to couple as much pump power as possible which results in higher levels of amplification in the DCF.
More recently, an improved side pumping scheme has been proposed and is disclosed in the PCT patent application publication WO 96/20519 to Gapontsev et al., published on Jul. 4, 1996. Publication WO 96/20519 illustrates the coupling arrangement between a multimode pump fiber and a DCF fiber functioning as a fiber amplifier. A multimode light source is coupled into the multimode fiber which has a tapered portion that is partially warped around an exposed portion of the inner cladding of the DCF and is thereafter fused to the inner cladding forming a monolithic integrated tapered structure with the multimode pump fiber integrated longitudinally along the length of the double clad fiber. This configuration is an improvement in side coupling over U.S. Pat. No. 4,815,079 since the multimode pump fiber is integrated into the inner cladding of the double clad fiber. However, as seen in figures of the publication, the multimode light enters the DCF at an angle which has to be less than the NA of the fiber so that, while improved light coupling is achieved, full advantage of the fiber NA is not achieved and, therefore, the coupling efficiency is not as high as in the case where if the multimode pump light could be direct coupled coaxially into the end of the DCF. However, as recognized by those in the art, taking this full NA advantage has not been possible without the use of the free-space approach, described above so that the single mode core of the DCF has to be, in some manner, interrupted in order to permit the end coupling of the light into the DCF inner cladding. Thus, the problem is how to accomplish such end pump coupling to take full advantage of the fiber NA without breaking or otherwise interrupting the DCF core carrying the propagating signal light.
Thus, pumping of DCFs is preferably based on end pumping of the double clad fiber rather than side pumping schemes as discussed above. Such end pumping schemes are favored over side pumping schemes because they can entertain the full NA of the input to the DCF. FIG. 2 illustrates a currently employed end pumping scheme 20 for a DCF, such as in the case of a 9 Watt, Yb doped double clad fiber laser. In FIG. 2, multimode output 26 from a standard pump laser diode bar 22, available from SDL, Inc. of San Jose, Calif., provides, for example, a 17 Watt output power which is delivered, via multimode fiber 24, for collimation and focusing into Yb core doped, DCF 29 using two discrete lenses 27A and 27B. Fiber 29 is provided with cavity mirrors 28A and 28B which are transparent to the pump wavelength or multiple wavelengths but highly reflective of the lasing wavelength within the absorption band of Yb. The NA of multimode fiber 24 is approximately 0.12 whereas the NA of fiber 29 is 0.47. Direct 1:1 imaging of fiber 24 to fiber 29 would not utilize the full NA of fiber. However, by demagnifying the output from the fiber coupled pump laser diode bar by a factor of three to take full advantage of the DCF NA, a much smaller double clad fiber can be employed.
The double clad pump configuration of FIG. 2 meets several of the DCF pump coupler requirements including efficient pump injection, pump brightness conservation and compatibility with a variety of standard pump sources. Over 90% of the pump light can be routinely injected into double clad fiber 29. The output format of fiber coupled pump laser diode bar 22 may be formatted to match any cross-sectional shaped of DCF 29 and the NA match can be easily achieved with lens system 27A and 27B. However, there are several important limitations which limit the end pumping schemes of both FIGS. 1 and 2. First, the single mode to single mode coupling of the signal light is relatively lossy because of the insertion loss of the lenses and dichroic mirror and tight, sub-micron mechanical tolerances for the single mode core alignment. The achievement of 90% coupling is likely, therefore, a maximum upper limit.
Second, the critical mechanical stability of the coupling scheme is made even more difficult by the injection of the high power pump light which may cause heating of the fiber input, possibly resulting in misalignments among the several optical components utilized to achieve optical beam coupling.
Third, the bare, exposed fiber end of either the signal or pump fiber or the DCF may become contaminated. The contamination may not only arise from outside the system but may also be caused by outer fiber cladding material or epoxies creeping over the end of the fiber. These effects have been observed and require careful fabrication particularly of the input end of the DCF. Contamination of the fiber input end core area where power densities are very high will result in fiber core damage with resulting coupling loss or even complete failure of the system.
Fourth, dichroic mirrors are difficult to fabricate. An example is 980 nm pumping of a Yb doped fiber providing an 1060 nm output, requiring high transmission of the 980 nm wavelength requiring a dichroic mirror capable of handling high transmission of the 980 nm wavelength through the mirror and high reflection of the 1060 nm at the mirror. This is accomplished with careful coatings applied to the mirror which must be fairly precise and results in an expensive optical coupling component.
Thus, as indicated previously, the problem then is how to accomplish such end pump coupling to take full advantage of the fiber NA without breaking or otherwise interrupting the DCF core carrying the propagating signal light. The pump light, which is multi-spatial mode, i.e., is distributed over a range of angles across the core of the multimode pump fiber, must be coupled with high efficiency into the inner cladding of the DCF. The multimode coupling could include a free-space coupling, since multimode couplings are much more tolerant of and robust against misalignment. However, the single mode core must be preserved unbroken or uninterrupted. Furthermore, for the end coupling schemes to be commercially viable, they should be compact, robust, and of low cost, as well as capable of accommodating a variety of different double clad fiber dimensions. Depending on the application, the end coupling scheme should readily accommodate different cross-sectional contours of the inner cladding of the DCF, which may be, for example, circular, noncircular, rectangular, or even star-shaped (i.e., a annular contour having a wavy perimeter or irregular boundary surface). The coupling technology must be able to accommodate any of these different fiber shapes. Also, the coupling technology must be of low insertion loss and thermally stabilized. This latter requirement eliminates many of the coupling technologies employed today for use in commercial telcom modules and applications. Typical pump powers are 1 W to 20 W per module. Even if there is a 0.5 dB (10%) loss in the coupling, up to 2 W of generated heat will occur in the optical coupler, which heat must be dissipated from the coupler without introducing misalignment or other forms of mechanical or optical loss.
Thus, a principal object of this invention is to provide optical coupling of a multimode fiber to a fiber that has a single mode core without interrupting or otherwise interfering with the integrity of a fiber""s single mode core, multiple single mode cores or multimode core.
A further object of this invention is to provide for more efficient coupling of pump light from a delivery fiber into the inner cladding of a DCF without interrupting or otherwise interfering with the single mode core of the DCF.
Another object of this invention is to provide end coupling of multimode pump source via a multimode fiber coupled into the inner cladding of a DCF without breaking or otherwise interrupting the single mode core of the DCF.
According to this invention, apparatus for coupling light into a light propagating medium within an optical waveguide comprises an optical fiber having an inner cladding of a predetermined refractive index surrounded by an outer cladding of lower index than the predetermined refractive index forming the optical waveguide and an optical coupler medium, having a refractive index that is substantially the same as the predetermined refractive index, in optical contact with an exposed portion of the inner cladding. The coupler medium has a body portion extending transverse to the longitudinal extent of the fiber for permitting the imaging of light external of the fiber into the inner cladding. The point of imaging in the inner cladding is where the light will be waveguided within the inner cladding of the fiber. Another version of the coupler includes a reflector which functions to reflect light propagating in the inner cladding back into the inner cladding of the fiber.
One version of an optical coupler for coupling light into an optical fiber comprises a double clad fiber (DCF) having a single mode or multimode core with a refractive index n1, an inner cladding with a refractive index n2, and an outer cladding with a refractive index n3, wherein n1 greater than n2 greater than n3. The couplers of this invention may also be employed for coupling light into multiple cores employed in an optical fiber. In any case, the outer cladding of the fiber is removed from a portion of the fiber exposing the surface of the inner cladding. An optical coupler, comprising glass or other optical material, having a refractive index substantially equal to n2 of the inner cladding, is formed, surrounding at least a portion, if not all thereof, of the exposed inner cladding portion of the fiber forming a bulk optical block that is optically transparent to and may be integrated with the optical material comprising inner cladding. The fiber exposed portion embedded within the coupler may be curved in an arc and is embedded within the optical bulk material. The embedded portion of the fiber may, therefore, be bent laterally, relative to an longitudinal extent of the fiber exiting the coupler, with the embedded portion of the fiber entering the coupler laterally from its side. The fiber radius of curvature in the coupler block is not so small as to bring about significant coupling losses of light out of the core. Thus, the fiber inner cladding in the embedded portion of the fiber within the coupler bulk becomes part of the coupler and the fiber core remains continuous and intact from its point of entry into the coupler and its point of exit from the coupler so that the fiber core remains uninterrupted from its point of origin, through its curved path within the optical coupler block, and onto its point of final destination. Thus, full advantage of the fiber NA is achieved with the signal fiber remaining uninterrupted and, therefore, the coupling efficiency of the pump light is maximized through direct coupling of the pump beam coaxially into the inner cladding of the fiber.
While the present invention pertains to coupling light into an optical fiber waveguide region, for purposes of explanation, reference in the embodiments will be made generally to DCFs having single mode cores. However, it should be realized that the invention is equally applicable to DCFs that have a multimode core or a plurality of single mode cores or to a multimode fiber with a multimode core.
A light source, such as pump light source, is aligned with the longitudinal extent of the fiber exiting the optical block in a manner to direct the light beam through the optical coupler directly into the end of the fiber inner cladding of the fiber at its point of exit from the coupler block. The coupler may also include a lens to converge and focus the pump light beam into the exposed end of the inner cladding within the optical block at the point of exit of the fiber from the coupler. The direction of the light beam is, therefore, transverse to a portion of the core of the DCF potion embedded in the coupler block. The higher index core of the fiber is, therefore, in part, in the path of the directed pump beam. However, since the core diameter is comparatively of a small size, i.e., only a few microns in diameter, the embedded core will not significantly interfere or obstruct the passage of the pump light beam into the inner cladding of the DCF.
Thus, an optical coupling for the pump light is provided by removing a portion of the outer cladding of a DCF and form a material bulk at the removed portion that is refractive index matched to the inner cladding rendering the pump light no longer confined to the inner cladding. Then, simple optics can be employed to reimage the pump light onto a point of the DCF outer cladding where it exits from the coupler.
The optical coupler medium of this invention performs the same function as the lenses and dichroic beamsplitter in the wavelength division multiplexer illustrated in FIG. 1. However, the coupler of this invention combines the pump and signal beams on the basis of their spatial, rather than spectral modal properties. For this reason, we refer to the coupler as a fiber-space-division-multiplexer, or FSDM. The FSDM coupler embeds the DCF within a structure that is index-matched to the inner cladding. External optics may be employed to couple pump light from a pump light source directly into the end of the fiber inner cladding without significantly perturbing the fiber core. Over the revised portion of the fiber that includes the FSDM coupler, the cladding is essentially unguided and light can freely propagate since the outer cladding has been removed, but the coupler forms a larger optical entrance for the pump beam to be coupled directly, end-wise, into the inner cladding of the DCF. A signal light is coupled into the core and remains confined to the core through the fiber as well as through the coupler. Within the index matching block, the fiber is bent through a shallow radius, for example, about a radius of 1 cm, to shift the pump light away from the optical path of the signal light. The pump light is focused in the coupler to a point of the inner cladding of the DCF where it exits the optical coupler. From this point forward, the pump light remains confined to and within the inner cladding of the DCF.
The utility of the robust FSDM coupler of this invention provides the last and most important component for realizing multi-functional laser configurations requiring no external single mode alignments, permitting all different kinds of laser configurations to be realized with double clad fiber laser technology. The FSDM coupler of this invention permits the simplest of configurations to be realized with the adaptation of a fiber laser source arbitrarily scaled in power through the provision of cascaded multiple pump modules added along the length of a single double clad fiber as well as configurations that comprise pulsed, Q-switched and MOPA architectures. Also, the coupler may be employed in cascaded amplifier configurations. Reflector couplers may also be employed in these configurations to counter-reflect unused pump light without interrupting the integrity and continuity of the laser or amplifier DCF core.
Other embodiments are enclosed that utilize the coupler structure as a means for removal of pump light via the coupler and out of the double clad fiber inner cladding for purposes of protecting optical components downstream from the pump light or for monitoring the intensity of the pump light in the inner cladding or the signal light in the core.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.